1. Field Of The Invention
This invention relates to the metal working art, and more particularly, to a method for stamping blanks from flat metal stock and to the stamped blanks which have accuratelyformed burr-free, peripheral surfaces.
2. Description Of The Prior Art
In conventional stamping or blanking operations, there is ever present the tendency for the material adjacent the blank periphery to be drawn axially over a punch diameter so as to present an uneven edge or rough margin, commonly referred to as a "burr", formed during the blanking operation. Such blanks are not suitable for use where surface smoothness is especially desirable as, for example, where sliding surfaces are to be encountered or proper seating is important. This is true of articles produced by stamping from flat metal stock such as washers, bearing retainers, and the like. To this end, the present invention contemplates a method whereby burr-free blanks conforming with predetermined dimensional and surface contour standards can be produced successfully from a strip of flat metal stock.
There have been attempts heretofore to produce burr-free and smooth, peripheral surface blanks stamped from flat metal stock. However, the equipment utilized has either been expensive, impractical, or failed in its intended purpose.
For example, U.S. Pat. No. 2,674,780 issued to M. H. Nielsen recognizes the problem that in conventional stamping or blanking operations there is ever present the tendency for the material adjacent the blank periphery to be drawn axially so as to form a burr. In order to solve this problem, the patentee provides a method and apparatus wherein successive blanks punched from flat metal stock are pushed through a series of stacked dies provided with teeth of varying length about their inner periphery through which the cut metal is pushed to lap, burnish and sever any burrs on the outer surface of the cut blank.
U.S. Pat. No. 2,636,253 issued to C. A. Rees illustrates a tool and method for removing burrs extending into an interior opening in a piece of metal by subjecting the burr to cold working and ultimately breaking the burr from the blank by passing the blank about a tool having a plurality of successively larger diameters which contact the burr to cold work and remove it from the stock surrounding the interior opening of the blank.
Another method utilized by the prior art in an attempt to produce burr-free blanks is known as "fineblanking". In this process, a blank is punched or stamped out of a piece of flat metal stock while the surrounding metal is compressed toward the punch to prevent the material from fracturing. However, while the resultant blank has shiny smooth edges throughout the length of its periphery, a burr is still formed adjacent one end of the peripheral edge as the metal material is drawn laterally and axially about the punch as the metal is pressed downwardly.
U.S. Pat. Nos. 1,476,706; 2,508,758; and 3,878,746, all disclose attempts at forming a burr-free blank by partially cutting a piece of stock from one side and then cutting the stock completely through from the opposite surface. While these patents suggest cutting the edge of a blank partially from each side and then causing an uncut portion intermediate the cut edges to fracture to sever the blank from the stock. Nevertheless, the punch and die configurations disclosed are such that when the lower punch cuts and severs the blank completely and pushes it out of the stock, an edge of the blank still can be drawn around the punch to form a burr. There is no recognition in these patents that the first cut opening must be larger than the second in order to assure that a controlled fracture occurs between the off-set cut edges, rather than a cut completely through the stock.
It has also been determined that a process which cuts along one edge, rather than off-set edges cannot achieve a true burr-free blank for the reason that the metal around the cut blank would shear, gall and tear up the blank edge as it was pushed from the stock. The use of a first cut, larger opening therefore, also enables the severed blank to be pushed freely from the metal stock without contact with the stock to reduce the possibility of having its edge snag to form a burr.
A different process is disclosed in U.S. Pat. No. 3,583,226. In this patent, a burr-free blank is formed by punching the metal stock from one surface, while simultaneously cutting the metal from its opposite surface. In lieu of cutting the stock completely through, a separate knockout member or ejector is used to fracture, rather than cut, the metal between its partially cut upper and lower portions to preclude the formation of a burr. While this procedure ostensibly will produce a burr-free article, the snagging of the blank on the surrounding metal still may occur and a complicated press and die arrangement utilizing a separate element is required, other than the die punches to sever the blank from the metal stock. Further, the metal fractured must be soft material only, so that the apparatus is effective.
The present method and apparatus overcomes the deficiencies noted in the prior art and also provides an inexpensive, yet highly successful method and apparatus to produce a truly burr-free stamped blank held to close tolerances.